Field of the Invention
The invention relates to an apparatus and a process for producing pigment-containing, in particular carbon-black-containing, polymer mixtures comprising polycarbonates and optionally elastomers, and/or other components, where the mouldings produced therefrom have, after the shaping process via injection moulding or via extrusion, improved surface properties and improved mechanical properties. The pigment-containing, in particular carbon-black-containing, polymer mixtures can be produced in a single processing step via simultaneous mixing in the melt of all components, without use of pigment masterbatches premixed in the melt. The components here are mixed in the melt in a corotating twin- or multiscrew extruder under non-aggressive temperature conditions, where, in the region from the plastifying zone as far as the die plate, the ratio between external screw diameter and internal barrel diameter, and also the value of the sum of external screw diameter and screw root diameter divided by twice the axial separation are smaller for the screws of these extruders than disclosed in the prior art (e.g. in DE 10 2004 010 553 A1).
Description of Related Art
When pigment-containing, in particular carbon-black-containing, polymer mixtures comprising polycarbonate and optionally elastomers, e.g. particulate rubbers, in particular particulate graft rubbers, are produced, the mouldings produced via injection-moulding processes or extrusion processes from these polymer mixtures often exhibit surface defects attributable to insufficient dispersion of the pigment particles, to a combination of insufficiently dispersed pigment particles and elastomeric components, or to elastomers that have not been uniformly dispersed. The average diameter of surface defects of this type due to agglomerates of pigment particles and/or due to elastomers is from 10 μm to 300 μm. The size of the surface defects can be determined by way of example by means of a reflected-light microscope. By way of example here, the surface is examined through an objective with magnification of 2.5 in bright-field mode using illumination by a halogen-100 light source, and suitable measurement equipment (e.g. image-evaluation software or a scale on the objective) is used to measure surface defects detected. The average diameter is defined as the length calculated by averaging the maximum diameter longitudinally and the maximum diameter transversely. The longitudinal direction is the direction in which the length of a particle is greatest. The said surface defects produce poor appearance, which is discernible by the naked eye and is perceived as unsatisfactory in numerous applications of these coloured, in particular black-coloured, polymer mixtures. Furthermore, if the agglomerates of the pigment particle and/or of elastomers exceed a certain size, mechanical properties are impaired, for example impact resistance or tensile strain at break.
The production of carbon-black-containing polycarbonates, and the occurrence of surface defects caused by aggregates of carbon during the processing of the said polycarbonate mixtures to give foils, are known and are described in JP 2008094984. This problem with the surface is solved by selecting suitable particle sizes for the polycarbonate and suitable densities for the carbon black when producing the mixture. However, no information is given about technical measures during mixing in the melt (compounding) of the components.
U.S. Pat. No. 6,180,702, too, describes polycarbonate mixtures in which carbon black and elastomers can be present, and which are free from surface streaking. The said polymer mixtures, which are produced by conventional processes, with the aid of customary extruders, comprise specific additives aimed at avoiding the surface defects.
WO 2004/094128 describes the production of polymer mixtures made of polyarylene ethers and of polyamides with block copolymers as impact modifiers and with carbon black in customary extruders which have different length-to-diameter ratios in the two different melt-mixing zones.
The distributive dispersion of carbon black masterbatches in high-density polyethylene (HDPE) is described in the publication “Plastics Processing Technology Online-News Letter”, 4 Jan. 2001, No. 0011 E, from “The Japan Steel Works, Ltd.”. Better distribution and dispersion of the carbon black masterbatch in the HDPE is achieved by enlarging the gap between the flight lands of kneading elements and the wall of the barrel specifically in the mixing zone and the barrel-wall part of a twin-screw extruder. In the said publication from “The Japan Steel Works, Ltd.”, the carbon black has been previously dispersed within the masterbatch, and this solution is therefore not transferrable to the use of pure, unpredispersed carbon black powder, where this is also an object of the present invention. Furthermore, the effect of additional elastomer components, and of other polymers, on the homogeneity of the mixtures remains unclear.
In the paper “Effect of kneading block tip clearance on performance of co-rotating twin-screw extruders”, in “ANTEC-Conference proceedings, 1999, Vol. 1”, the “Mixing” section on p. 222 states that in the case of HDPE-carbon-black-masterbatch mixtures enlarging the gap between the flight lands of kneading elements and the barrel wall of a twin-screw extruder impairs the dispersion of the carbon black masterbatch in the HDPE matrix when comparison is made with narrower gaps. In the light of the said publication, a person who is skilled in the art and who is facing the problem to be solved in the present invention would therefore have no interest in enlarging the gap.
In the prior art, for example in DE 10 2004 010 553 A1, gap widths between screw flight land (external screw diameter) and barrel wall section (internal barrel diameter) of less than 1% are stated, corresponding to an external screw diameter:internal barrel diameter ratio of more than 98.4%, and the difference between screw flight land (external screw diameter) and root (screw root diameter) of an adjacent screw element is stated to be at most 1% of the bore diameter, and the corresponding figure for the sum of external screw diameter and screw root diameter divided by twice the axial separation is 99.8%. This disclosure differs from the prior art described in that screw-based machines have screws having gap widths, between screw flight land and barrel wall section, of from 2% to 10% of the bore diameter, and a root clearance of at most 1% of the bore diameter between a screw root and a screw flight land. The said screw elements are intended to be used for shear-sensitive products, since these screw elements permit compounding at non-aggressive temperatures. However, a person skilled in the art would conclude from this information that there can be no improvement in dispersion, since less energy is introduced. Nor is anything actually said about any effects of these screw elements on the quality of dispersion and the homogeneous dispersion of infusible additives. This type of information relating to gap widths is very general in nature and not readily transferrable to filler-containing or elastomer-containing polymer mixtures.
Since no satisfactory measures for solving the present problem can be derived from the prior art, suitable measures were sought for achieving comminution, and homogeneous dispersion within the polymer matrix, of pigment particles, in particular carbon black, during mixing with compounded thermoplastic materials, preferably compounded polycarbonate materials, optionally comprising elastomers, preferably elastomer-containing compounded polycarbonate materials, under non-aggressive temperature conditions in such a way that no significant surface defects occur on the mouldings produced from these mixtures. Surface defects are unevenness phenomena directly on the surface of the moulding. The unevenness phenomena can be either depressions or elevations, where these increase the roughness of the surface. The average diameter of the depressions and, respectively, elevations is usually from 10 μm to 300 μm, where 10 μm is the lower limit of resolution of the measurement method used. This means that smaller surface defects can also occur. The depth of the depressions and, respectively, the height of the elevations is from 50 nm to 100 μm. The depth of the depressions is determined by CLSM (confocal laser scanning microscopy) topography.
A problem addressed by the invention was therefore to produce polymer mixtures comprising polycarbonate, optionally elastomers, and pigments, in particular carbon black as pigment, and also optionally further additives and thermoplastics, in which the components mentioned have excellent dispersion within the polycarbonate matrix and have such uniform distribution that mouldings obtained via injection moulding or extrusion of the said polycarbonate mixtures have a defect-free surface, and also improved mechanical properties. By virtue of the process according to the invention, and use of the apparatus according to the invention, the surface of the moulding has a smaller number of defect sites larger than 10 μm, where agglomerates of insufficiently dispersed pigment particles and/or elastomers are responsible for these defect sites. Another problem addressed by the invention was the compounding of the abovementioned components in a single melting step in tightly intermeshing, corotating twin- or multiscrew extruders with minimum energy input with avoidance of local temperature peaks and preferably without use of previously compounded pigment masterbatches, e.g. carbon-black masterbatches.